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Many people know about the Shepard-Risset Tone/Glissando, which is a tone that starts low and seems to continue ascending(or descending) when you listen to it, even though it actually is playing the same thing over and over again.

Wikipedia has a nice article on this.

However, I read that Risset also made another sound illusion: The so-called Risset Rhythm, a drum loop that seems to keep on going faster and faster (Wikipedia does have an example of this, but no explanation). However, I'm unable to find any explanation of how this illusion works.

Could somebody explain it to me please?

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It's a self-similar fractal. –  Mechanical snail Jan 2 '13 at 23:56
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up vote 6 down vote accepted

Sounds to me like exactly the same principle.

The first rhythm gets faster and faster until it becomes a blur of noise and is removed from the sound, but over the top of that is superimposed the same rhythm at half speed. While you're listening to the first rhythm get faster, the second does the same, and eventually becomes the main focus of attention. By the time it does so, it's probably got to the point where you can just about hear it again at half speed.

The same piece of audio can contain both the original rhythm and half-speed version, so gets looped to create the illusion.

As you notice the rhythm getting faster, listen out for the same features, but in half-time.

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Yep, definitely seems to be the same principle. I can hear the same thing going on; in fact, I could concentrate and isolate the slower rhythms fairly easily in this recording: swiki.hfbk-hamburg.de:8888/MusicTechnology/uploads/826/… –  Josh Fields Nov 27 '11 at 4:15
    
It sounds as if the volume of the rhythm is a bell curve function of it's closeness to "correct" speed. So, it fades in at approx half speed, reaches max volume at full speed, then fades out at approx double speed, always mixed in with its previous iteration. –  slim Nov 28 '11 at 9:59
    
Plus as the original gets faster and faster it gets shorter and hides (or blends) into the backbeat of the slower one so it's less and less audible. Not to mention that the faster sound exits as a kind of octave higher of the original snare accent. –  percusse Jan 4 '13 at 2:04
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